I had thought that mixing two signals together was dead easy, simply merge them through R resistors at least times ten then input source impedance and then follow them with a buffer stage stage of at least R times 10. That means the buffer stage sees 50% of each signal (assuming two sources) and additionally as R is times 10 the input source's impedance, the voltage drop across it will almost eliminate any back feed from one source into another.

That is how I understand it in my amateurish thinking.

Looking at a few reverb schematics (one from my Fender princeton chorus amp), it appears there are more issues at play than how I described it above. It seems there is some play with "ground level" and I am not going to try and describe it because I will surely mess up.

Could someone please explain it to me in simple terms and big pictures?

If I'm looking at the right schematic(s), I see just a simple inverting opamp stage using two inputs, hence becoming a "mixer". e.g.
Roughly, the ratio of the input resistor and feedback resistor defines the gain per each input. I suppose the signal can't bleed to the other inputs either since the inverting terminal is effectively a virtual ground.